Variant H3N2 Influenza Virus: What You Should Know

Michael Jhung, MD, MPH


February 06, 2012

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Hello. I am Dr. Michael Jhung. I am a medical officer in the Epidemiology and Prevention Branch of CDC's Influenza Division. I am happy to speak with you today as part of the CDC Expert Video Commentary Series on Medscape. You may have heard about recently reported cases of human infection with a new influenza A H3N2 virus being called "variant H3N2," or "H3N2v," for short. Today I will review what we know about this virus and cases reported as of the end of 2011.

In the last 6 months of 2011, 12 US residents in 5 different states (Indiana, Iowa, Maine, Pennsylvania, and West Virginia) were found to be infected with this influenza A H3N2 variant virus that had genes from avian, swine, and human influenza viruses. Similar viruses have been found in the past in swine but only very rarely infected people. However, this virus, first detected in August, was a little bit different. It contained another genetic change: It had acquired the matrix (M) gene from the 2009 H1N1 pandemic virus.

Although we know that the M gene plays a role in viral assembly and replication, we aren't certain what the addition of this M gene means in terms of illness severity or transmissibility in humans. Similar H3N2 viruses have circulated in US swine since the 1990s, and the H3N2 variant with the 2009 H1N1 pandemic M gene was detected in US swine as early at 2010. August 2011, however, was the first time this virus was found in a person.

Investigations into the 12 cases have shown human infection following contact with swine, as well as limited human-to-human transmission. So far, most illness associated with this virus has been mild and self-limited. However, although all the patients recovered, 3 were hospitalized, all of whom were people with preexisting medical conditions. All but 1 were young children. Limited serologic studies conducted recently at CDC indicate that adults may have some preexisting immunity to H3N2v, whereas children likely do not.

Although there is no evidence that sustained human-to-human transmission of H3N2v is occurring, CDC is watching this situation very closely. All influenza viruses have the capacity to change, and it is possible that this virus will continue to cause illnesses in humans, either from exposure to infected swine or from subsequent human-to-human transmission. Like seasonal flu, certain people are likely at greater risk for serious complications from infection with this virus. As a result, cases of severe illness, even deaths, resulting from infection with H3N2v could occur.

Because infections with variant influenza A viruses have been identified both after exposure to pigs and after limited human-to-human transmission, CDC is recommending that clinicians consider the possibility of infection with one of these variant viruses when seeing patients with febrile respiratory illness. This is particularly true when the patient has been exposed to pigs and/or if the patient is a young child in a state where infections with variant influenza viruses have occurred. As usual, if influenza is suspected, CDC recommends empiric treatment with the influenza antiviral medications oseltamivir (Tamiflu®) or zanamivir (Relenza®) for patients who are severely ill or who are at high risk for serious flu-related complications. Information about H3N2v is being constantly updated on the CDC Website. New guidance has recently been posted related to specimen collection, processing, and testing for patients with suspected H3N2v infection.

Although CDC's emphasis right now is on rapid detection and treatment, a candidate vaccine virus to protect against H3N2v has been prepared by CDC and provided to World Health Organization Collaborating Centers and vaccine manufacturers should production of a vaccine become necessary. The seasonal flu vaccine is unlikely to protect against flu viruses that normally circulate in swine.

For more information about H3N2v, please visit the CDC Website.

Thank you.

Michael Jhung is a Medical Officer in the Influenza Division at the Centers for Disease Control and Prevention (CDC) where he regularly conducts surveillance, outbreak investigation, and pandemic preparedness and response activities. He received his BS (1989), MS (1994), and MPH (2004) degrees from the University of Michigan and his MD (2002) from Case Western Reserve University. Dr. Jhung maintains professional interests in communicable disease epidemiology, infection control in resource-limited settings, and the public health response to disasters and emergencies. Dr. Jhung is board certified in general preventive medicine and public health and is a 2007 graduate of the Epidemic Intelligence Service program at CDC.


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